1. Field of the Invention
This invention relates generally to DC-to-DC converters and, more particularly, to converters of the blocking oscillator type wherein standby losses are minimized.
2. Description of the Prior Art
In telephone system applications, particularly those relating to transmission over multipair cables, deleterious interference by spurious signals causes noise and degrades the quality of transmission. Oftentimes the unwanted signals are electromagnetically coupled to the individual pairs comprising the cable and cause longitudinal or common-mode current to flow along the conductors of the pair to ground. If the cable pair itself or the pair terminations are not well-balanced or matched with respect to ground, the longitudinal current is converted to a metallic or balance-mode signal which results in degrading noise. Noise arising from such unbalances may be reduced by improving the balance, reducing the longitudinal current flowing through the unbalanced components or by a combination of both techniques.
Conventional noise mitigation techniques generally employ magnetic apparatus such as a repeat coil or a multipair neutralizing transformer. The repeat coil provides improved performance by increasing the balance at a termination point, but at the expense of manufacturing complexity in constructing precision components. The neutralizing transformer mitigates noise by providing a counteracting longitudinal voltage. However, the transformer technique has the disadvantage that all pairs to be treated by a single transformer must be in the same cable. Another disadvantage is that a single subscriber-induced longitudinal signal is coupled to the longitudinal circuit of all other pairs being protected.
An electronic alternative to the magnetic technique is the subject matter of this invention. Conventionally, at least one end of a subscriber loop or trunk pair is terminated in a low impedance, battery feed circuit having center taps connected to battery and ground. The pesent invention relates to a DC-to-DC converter, which has its output isolated from both the battery and ground and which is substituted for the conventional battery-ground circuit. Longitudinal current is reduced because the low impedance longitudinal ground connection is eliminated, thus providing a concomitant reduction in metallic noise arising from longitudinal-to-metallic conversion.
Also as part of the overall battery-ground substitution strategy of the present invention, since the converter is utilized in a system wherein powering requests are intermittent, it is necessary to minimize losses in the standby mode. Prior art converters do not provide the combined requirements of input-to-output isolation and minimal standby power loss.
U.S. Pat. No. 3,427,525, issued to J. C. Thornwall, is representative of prior art converters which are designed primarily to regulate the continuous output signal of the converter. Regulation in the circuit of Thornwall is accomplished by clamping and then releasing a blocking oscillator depending on the load power demands. The clamp release times are determined by a comparison of the output load voltage to a fixed reference voltage. This comparison is accomplished by a direct feedback connection between the load and comparator circuit. Such a connection destroys the isolation between input and output which is essential to increased common-mode impedance.